Catalytic filters are employed for a variety of fluid filtering applications. Typically, these filters combine a catalytic material (e.g., TiO2, V2O5, WO3, Al2O3, MnO2, zeolites, and/or transition metal compounds and their oxides) within a matrix. As the fluid passes over or through the matrix, target species within the fluid react with catalyst particles to convert the target species to a more desirable by- or end-product, and therefore remove the target species from the fluid stream. Examples of such catalysts include:
TABLE 1Example CatalystsTarget speciesActive MaterialResulting Product(s)NOx, NH3TiO2, V2O3, WO3N2 + H2OCOAl2O3, PtCO2Dioxin/FuranTiO2, V2O3, WO3CO2, HCl, H2OO3MnO2O2
Examples of previous attempts to produce a catalytic filter device include those set forth in U.S. Pat. Nos. 4,220,633 and 4,309,386, to Pirsh, where filter bags are coated with a suitable catalyst to facilitate the catalytic reduction process of NOx. In U.S. Pat. No. 5,051,391, to Tomisawa et al., a catalyst filter is disclosed which is characterized in that catalyst particles which are made of metal oxides with a diameter of between 0.01 to 1 um are carried by a filter and/or a catalyst fiber. In U.S. Pat. No. 4,732,879, to Kalinowski et al., a method is described in which porous, preferably catalytically active, metal oxide coatings are applied to relatively non-porous substrates in a fibrous form. In patent DE 3,633,214 A1, to Ranly, catalyst powder is incorporated into multilayered filter bags by inserting the catalyst into the layers of the filter material. Further examples to produce catalytic filter devices include those set forth in JP 8-196830, to Fujita et al., in which a micropowder of an adsorbent, reactant, or the like is supported in a filter layer interior. In JP 9-155123, to Sasaki et al., a denitrification layer is formed on a filter cloth. In JP 9-220466, to Kaihara et al., a catalyst filter is made by impregnating a cloth of glass fibers with titanium oxide sol which is then heat treated and further impregnated with ammonium metavanadate. In JP 4-219124, to Sakanaya et al., a compact, thick, and highly breathable filter cloth is filled with catalyst for the bag filter material in order to prevent catalyst separation. In U.S. Pat. No. 5,620,669, to Plinke et al., the filter comprises composite fibers of expanded polytetrafluoroethylene (ePTFE) having a node and fibril structure, wherein catalyst particles are tethered within the structure. U.S. Pat. No. 6,331,351, to Waters et al. teaches chemically active particles attached to a porous substrate by means of a polymer adhesive. A microporous layer is attached to at least one side of, or within, the porous substrate. The resulting filter material removes contaminants such as dust, from the filter stream before the dust can clog active catalytic sites, as well as remove undesirable species by means of catalysis or reaction.
During filter operation, two main problems typically can occur with the conventional constructions, namely chemical deterioration and mechanical deterioration. With chemical deterioration, the chemical function of the filter can be rendered useless due to contamination, which is a serious problem with virtually every conventional active filter device, and especially for catalytic filter devices. Although, by definition, catalysts are not consumed during the catalytic reaction, catalytic filters may have limited operating lives due to particle, liquid, and gaseous contamination from a fluid stream (i.e., fine dust particles, metals, silica, salts, metal oxides, hydrocarbons, water, acid gases, phosphorous, alkaline metals, arsenic, alkali oxides, etc.). Deactivation occurs because the active sites on the active particles within the filter are physically masked or chemically altered. Unless these contaminants can be shed from the filter, the filter will rapidly diminish in efficiency until it must be replaced. Additionally, in some instances, the processing aids used in manufacture can cause deterioration of the catalysts. A variety of cleaning apparatus exist to remove dust from filters (e.g., shaker filter bags, back-pulse filter bags and cartridges, reverse air filter bags, etc.), but these devices are not particularly effective at removing dust embedded inside the filter material.
Another form of chemical deterioration is due to the loss of inserted catalysts during operation. The catalyst particles in many instances are not attached strongly enough to the host fibers to withstand the rigors of normal operation. As a result, the catalyst particles fall out of the filter, thereby not only diminishing filter effectiveness, but also contaminating the clean fluid stream.
With respect to mechanical deterioration, the mechanical function of the filter can deteriorate by abrasion of the filter fibers during operation or by the penetration and collection of dust contaminates in the filter. Another mechanical failure is due to dust particle break-through. Additionally, high temperature (e.g., at least 160° C.) operation and reactive chemical species in typical filtration systems and bag houses can cause deterioration of the filter media over several years or, in some cases, several months.
Japanese Patent Application No. 10-230119, assigned to ABB Co., Ltd., is directed to a filter material formed by immersing fibers to be formed into a filter cloth in a liquid catalyst, drying the catalyst, molding the fibers into a filter cloth and applying an ethylene tetrafluoride resin continuous porous thin film to the filter cloth. U.S. Pat. No. 5,620,669, described earlier, combines the concept of catalyst particle protection by a microporous membrane and the incorporation of catalyst particles attached directly to nodes and fibrils for strong adhesion and low pressure drop. U.S. Pat. No. 6,331,351 described earlier, addresses many of the chemical and mechanical challenges described. However, improved performance is still needed.